Enzyme immobilisation on planar and porous silicon substrates for biosensor applications

Two methods for the immobilisation of enzymes on silicon-based so-called el ectrolyte-insulator-semiconductor (EIS) structures are suggested. These EIS structures are used as a basis for potentiometric biosensors. In the first method, heterobifunctional cross-linker molecules are employed to covalent ly bind enzymes to these capacitive layer structures which possess a planar surface that contains amine groups. Porous EIS sensors which, in compariso n to planar sensors, exhibit an enlarged surface area, are used in the seco nd method. For the first time, pH-sensitive Si3N4 was deposited on the wall s and bottoms of the SiO2-covered pores. Here, a large amount of enzyme mol ecules can adsorptively be bound inside the porous structure. Penicillinase is used as a model enzyme. Capacitance-Voltage and Constant Capacitance me asurements are performed in order to examine the respective penicillin sens or responses and thus to validate both immobilisation methods. Whereas the sensitivity of the sensors prepared by both methods is nearly identical for low penicillin concentrations up to around 0.25 mM, a difference of the ca libration curves in the higher concentration range indicates a larger amoun t of immobilised enzyme in the case of the porous structures. (C) 1999 Else vier Science B.V. All rights reserved.